The turnover rate of acetylcholine was studied in rats injected intraventricularly with prolactin or exposed to cold for different periods of time. Prolactin increased the turnover rate of acetylcholne selectively in the striatum and hippocampus and this effect was prevented by the previous destruction of dopaminergic pathways with 6-OHDA (injected in the A9-A10 areas 10 days prior to the experiment). Exposure to cold for 1 and 4 hours selectively decreased the turnover rate of acetylcholine in the frontal cortex by about 40 per cent. Turnover rates of acetylcholine in brain areas of rats exposed for 24 hours to cold were unchanged.